1. Field of the Invention
The present invention relates to an output amplifier circuit, which includes an instrumentation amplifier for amplifying a small differential voltage output from a sensor such as an acceleration sensor and an offset voltage adjustment circuit, and a sensor device using the output amplifier circuit.
2. Description of the Related Art
An example of an output amplifier circuit including an instrumentation amplifier having an offset adjustment capability is illustrated in the description and FIGS. 1 and 6 of Japanese Unexamined Patent Publication Kokai No. 2003-215172 which is entitled “CHARGE/DISCHARGE CURRENT DETECTION CIRCUIT AND VARIABLE RESISTIVE DEVICE” and is hereinafter referred to as a “first patent publication”. An example of a sensor device for amplifying small differential output voltages output from an acceleration sensor through an output amplifier circuit having an instrumentation amplifier is illustrated in the description and FIGS. 4 and 7 of Japanese Unexamined Patent Publication Kokai No. H7-244071, which is entitled “SENSITIVITY ADJUSTMENT DEVICE FOR ACCELERATION SENSOR” and is hereinafter referred to as a “second patent publication”.
The output amplifier circuit described in the first patent publication is composed of an instrumentation amplifier and an off set voltage adjustment circuit. The instrumentation amplifier amplifies a small differential voltage output from a bridge circuit or the like, and requires a high gain of, for example, several tens to several thousand fold, a low drift, and a low offset. Accordingly, the instrumentation amplifier of the first patent publication amplifies small differential output voltages from detection resistors through first and second operational amplifiers (hereinafter, also referred to as an “Op-amp”), and switches between the output voltages of the first and second Op-amps using a switch, and further amplifies the switched output voltage through a third Op-amp to output an amplifier circuit output voltage. In addition, an offset voltage adjustment circuit is provided to adjust the offset error of the third Op-amp.
The sensor device described in the second patent publication is composed of an acceleration sensor including a Wheatstone circuit using piezoresistive elements and an output amplifier circuit for amplifying small differential output voltages from the acceleration sensor through an instrumentation amplifier. The instrumentation amplifier, which constitutes the output amplifier circuit, amplifies the differential output voltages through first and second amplifiers, and amplifies the difference between output voltages of the first and second Op-amps to output an amplifier circuit output voltage.
The output amplifier circuit described in the first patent publication uses the offset-voltage adjustment circuit to adjust the offset voltage of the third Op-amp provided at the output stage. However, the offset voltages of the first and second Op-amps are different from each other, so that the Op-amp adjustment value for the third Op-amp when the first Op-amp is connected to the third Op-amp via the switch must be different from that when the second Op-amp is connected to the third Op-amp via the switch. This makes it difficult for the offset adjustment circuit to set the Op-amp adjustment value. This problem occurs also in the output amplifier circuit described in the second patent publication.
To solve these problems, one might propose, for example, in the output amplifier circuit of the second patent publication, that the voltage adjustment circuit be connected to the input of either the first Op-amp or the second Op-amp (for example, the input of the first Op-amp) and that the offset voltage of the first Op-amp be adjusted so as to make the output voltages of the first and second Op-amps equal.
However, in this configuration, for example, if common mode noise is carried in the differential output voltages from the acceleration sensor described in the second patent publication, the output voltage of the amplifier circuit may vary from a specified value, thereby worsening a Common Mode Rejection Ratio (CMRR).